The present invention relates to a phase locked loop that produces an output signal whose frequency is half of the frequency of the input signal to the phase locked loop.
A phase locked loop is typically implemented as an electronic circuit that controls an oscillator so that the oscillator maintains a constant phase angle relative to a reference signal. Such a phase locked loop may be used for coherent carrier tracking and threshold extension, bit synchronization, symbol synchronization, tape synchronization, modems, FSK demodulation, FM demodulation, frequency synthesizer, tone decoding, frequency multiplication and division, SCA demodulators, telemetry receivers, signal regeneration, and coherent demodulators. Such a phase locked loop can also be used in connection with angular rate sensors.
Angular rate sensors are used as components of navigational and inertial guidance systems for aircraft, spacecraft, ships, missiles, etc. Although mechanical gyroscopes were used in the past for angular rate sensing, ring laser gyros and vibrating quartz gyros have displaced mechanical gyros because ring laser gyros and vibrating quartz gyros have characteristics that are superior to those of mechanical gyros.
A particularly economical vibrating quartz gyro employs pairs of parallel tines. Such a quartz gyro is described, for example, in Fersht et al., U.S. Pat. No. 5,056,366 and in Staudte, U.S. Pat. No. Re 32,931. One pair of tines (the drive tines) is driven by an oscillator so that the tines move toward each other and away from each other. Rotational motion of the tines about a central longitudinal axis causes the vibration of the drive tines to couple, by coriolis force, to the other pair of tines (the pick-off tines). The coriolis force causes the pick-up tines to vibrate in such a way that, when one pick-off tine moves in one direction, another pick-off tine moves in the opposite direction. The force, which drives the pick-off tines, is proportional to the cross-product of the angular rate of rotation and the linear velocity of the drive tines.
The output signal from the quartz gyro appears as a double-sideband suppressed-carrier (DSSC) modulation of the input angular rate, where the carrier frequency is the frequency of oscillation of the drive tines. Therefore, an angular rate signal can be recovered from the output signal by a synchronous demodulator.
Analog circuits have been used for driving the quartz gyro and for synchronous demodulation of the output signal. Analog circuits, however, are subject to voltage offsets and component value drift due to temperature variations and aging. These problems are particularly troublesome due to peculiarities of the quartz gyro that are not apparent from the simplified or xe2x80x9cfirst orderxe2x80x9d operating characteristics of the analog circuit.
One such problem is related to the resonant frequencies of the drive tines and the pick-off tines. If the pick-off tines have the same resonant frequency as the drive tines, a maximum amplitude response is obtained from the pick-off tines. Thus, the signal to noise ratio is optimum. On the other hand, it is undesirable for the pick-off tines to have exactly the same resonant frequency as the drive tines because of the resulting non-linearity between the output angular rate signal and input angular rate that occurs due to the impact of pick-off tines dynamics on the output signal.
Accordingly, a compromise is usually achieved between the need for a more linear function and the need to avoid limiting the dynamic range due to noise. This compromise is achieved by providing a resonant frequency offset that is, to an extent, dependent on the bandwidth of the angular rate signal. In particular, the pick-off tines have a two-pole resonance characteristic, giving a second-order response far away from the resonant frequency.
In practice, these considerations dictate that the difference between the resonant frequency of the drive tines and the resonant frequency of the pick-off tines should be about twice the bandwidth of the angular rate to be sensed by the quartz gyro. A typical quartz gyro for inertial navigation applications, for example, has a difference of about 100 Hz between the drive resonant frequency and the pick-off resonant frequency. This difference in resonant frequencies causes the amplitude of the angular rate signal to be dependent on the frequency, as well as on the amplitude of vibration, of the drive tines. Moreover, the temperature dependence of the difference between the drive and pick-off resonant frequencies is the most critical temperature dependent parameter of the quartz gyro.
To obtain sufficient performance for inertial navigation, the analog circuits associated with the quartz gyro have been relatively complex and expensive. Moreover, it is estimated that the limitations of the prior art analog circuitry cause the performance of the quartz gyro to be about an order of magnitude less than that theoretically possible and attainable by sufficiently complex digital signal processing.
The present invention is directed to a phase locked loop that overcomes one or more of the problems of the prior art.
In accordance with one aspect of the present invention, an apparatus that implements a digital phase locked loop comprises an automatic gain control, a 90xc2x0 phase shifter, a phase detector, a loop filter, and a digital dual frequency oscillator. The automatic gain control applies gain to an input signal in order to produce a gain controlled signal. The 90xc2x0 phase shifter provides a 90xc2x0 phase shifted version of the gain controlled signal. The phase detector is driven by the gain controlled signal, by the 90xc2x0 phase shifted version of the gain controlled signal, and by sinusoidal and co-sinusoidal signals. The loop filter integrates an output of the phase detector and provides servo equalization for the phase-locked loop. The digital dual frequency oscillator has a fundamental frequency controlled by an output signal from the loop filter in order to generate the sinusoidal and co-sinusoidal signals.
In accordance with another aspect of the present invention, a method implemented by a phase locked loop comprises the following: applying a gain to an input signal to produce an in-phase gain controlled signal, wherein the input signal has a frequency 2f0; shifting the in-phase gain controlled signal by 90xc2x0 to produce a quadrature gain controlled signal; detecting a phase difference dependent upon the in-phase gain controlled signal, the quadrature gain controlled signal, and first and second output signals; and, producing the first and second output signals and a third output signal in response to the phase difference, wherein the third output signal has a frequency f0, and wherein the each of the first and second output frequencies has a frequency 2f0.
In accordance with yet another aspect of the present invention, a method of driving a gyro comprises the following: shifting an in-phase signal by 90xc2x0 to produce a quadrature signal, wherein the in-phase signal is derived from a first output of the gyro, wherein the in-phase signal has a frequency 2f0, and wherein the quadrature signal has a frequency 2f0; detecting a phase difference dependent upon the in-phase signal, the quadrature signal, and first and second output signals; producing the first and second output signals and a third output signal in response to the phase difference, wherein the third output signal has a frequency f0, and wherein the each of the first and second frequencies has a frequency 2f0; producing an angular rate indicating signal based upon a second output of the gyro and the first and second output signals; and, driving the gyro in response to the third output signal.